In recent years, in addition to wireless communications using electric waves or infrared (IR) rays, a communication using visible light such as indoor lighting devices, outdoor billboard illuminations, traffic lights, and car headlamps is at the center of attention. In particular, white light emitting diodes (LEDs) have been assiduously developed lately and applied to various fields such as illuminations, automotive lamps and backlight units (BLUs) of liquid-crystal displays. A white LED has a much higher on/off switching response speed than other light sources such as a fluorescent lamp. Thus, a visible light communication system utilizing a white light of a white LED as a data transmission medium which thereby provides a data transmission function to the white LED has been proposed.
FIG. 8 illustrates a typical configuration of a visible light receiving apparatus 100 of a conventional visible light communication system. As shown in FIG. 8, the visible light receiving apparatus 100 includes a photoelectric conversion unit 110 configured to convert a signal light transmitted by a transmitter into an analog signal, an amplifier 120 configured to amplify the analog signal to a certain signal level that may be recognized by a quantizer 130 of a subsequent stage, and the quantizer 130 configured to convert the analog signal amplified by the amplifier 120 into a digital signal that may be recognized by a digital circuit 140 of a subsequent stage. The digital circuit 140 may be, for example, a communication control circuit.
FIG. 9 illustrates a typical configuration of a photoelectric conversion unit 110 of a conventional visible light communication system. As shown in FIG. 9, the photoelectric conversion unit 110 includes a photodiode 111 serving as a light receiving unit and a resistor 112 connected to an anode of the photodiode 111 in series. A cathode of the photodiode 111 is connected to a positive power source Vcc, and the resistor 112 is connected to a ground GND. An amplifier 120 of a subsequent stage may be connected to a connection point of the photodiode 111 and the resistor 112 via a coupling capacitor 113. In the photoelectric conversion unit 110, a voltage [Vcc−Vr], where Vcc denotes the voltage of the positive power source and Vr denotes an inter-terminal voltage of the resistor 112, is applied as a reverse bias to the photodiode 111 so that a photocurrent Ipd corresponding to a light intensity of signal light flows through the photodiode 111. Thus, a reverse bias voltage Vpd applied to the photodiode 111 may be expressed as [Vcc−R×Ipd], where R denotes resistance of the resistor 112. The resistor 112 functions as a current-to-voltage (UV) converter which converts the photocurrent Ipd into a voltage and also functions as a bias resistor which applies a bias voltage to the photodiode 111. Furthermore, a low-frequency component may be blocked by the coupling capacitor 113 so that an AC component is applied as an optical signal to the amplifier 120 of the subsequent stage.
However, in most cases, ambient light such as sunlight or light emitted from an electric lamp for an illumination system or a fluorescent lamp as well as the signal light transmitted from the transmitter may be incident upon the diode 111 in visible light communication. Among the ambient light, the sunlight is detected as a DC component of the photocurrent Ipd of the photodiode 111. In addition, when a pre-emphasis or pre-bias process is employed, even the signal light transmitted from the transmitter may be detected as the DC component of the photocurrent Ipd. For these reasons, the DC component of the photocurrent Ipd of the photodiode 111 increases in the environment under a strong sunlight or in the environment close to the transmitter. As a result, the reverse bias voltage Vpd applied to the photodiode 111 decreases as a voltage drop due to the resistor 112 increases. Thus, failing to secure a reverse bias voltage required for operations of the photodiode 111 results in communication errors.
To solve this problem, a direct-current (DC) feedback method or an active bias method has been proposed. For example, the DC feedback method is disclosed in Japanese Patent Laid-open No. 2006-5599. FIG. 10 illustrates a basic circuit configuration of the DC feedback method. As shown in FIG. 10, in the DC feedback method, a light receiving unit 201 is biased by a voltage-controlled current source 202. An alternating current obtained by removing a DC component from an output of the light receiving unit 201 by a DC cut capacitor 203 is supplied to an amplifier 204. Furthermore, the voltage-controlled current source 202 is voltage-controlled by an output of the amplifier 204. Thus, a reverse bias voltage applied to the light receiving unit 201 is maintained constant.
The active bias method is disclosed in Japanese Utility Model Laid-Open No. 56-071643. According to the active bias method, a circuit is configured by replacing the resistor 112 of FIG. 9 with an element (e.g., a cadmium sulfide (Cds) element) whose resistance varies according to light intensity of signal light. Thus, since the resistance of the Cds element decreases with an increase in a DC component of the photocurrent Ipd of the photodiode 111, a reverse bias voltage applied to the photodiode 111 is maintained at a predetermined level or higher.
However, since the DC feedback method uses an active circuit, noise is increased. Moreover, since the DC feedback method uses a feedback control, a time constant of feedback decreases when the DC component of the photocurrent Ipd increases, resulting in an increase in a low-pass cut-off frequency. That is, the DC feedback method has a drawback that a frequency band of a signal is reduced with an increase in the DC component of the photocurrent Ipd. Furthermore, since a circuit using the DC feedback method requires a complicated configuration having a large number of components, cost reduction or miniaturization is precluded.
The active bias method involves allowing the signal light to be incident upon not only the photodiode 111 but also the Cds element. Thus, an arrangement of elements is extremely difficult, and a physical design thereof is very limited. In particular, since it is necessary to prevent communication conditions from affecting a difference in light receiving intensity between two elements, employing a circuit utilizing the active bias method as a product is practically undesirable.